1. Field of the Invention
This invention relates to suspension systems for automobiles and machines which receive mechanical shock, and more particularly to a method and apparatus for determining the displacement of a piston within a shock absorber.
2. Description of Related Art
Shock absorbers are used in connection with automotive suspension systems to absorb unwanted vibrations which occur during driving. To dampen unwanted vibrations, shock absorbers are generally connected between the body and the suspension of the automobile. A piston assembly is located within the shock absorber and is connected to the body of the automobile through a piston rod. Because the piston assembly is able to limit the flow of damping fluid within the working chamber of the shock absorber when the shock absorber is compressed or extended, the shock absorber is able to provide a damping force which "smooths" or "dampens" vibrations transmitted from the suspension to the body.
The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston assembly, the greater the damping forces which are provided by the shock absorber. Acccordingly, a "soft" compression and rebound stroke is produced when the flow of damping fluid in the working chamber is relatively unrestricted. In contrast, a "firm" compression and rebound stroke is produced when there is an increased restriction in the flow of damping fluid in the working chamber.
In selecting the amount of damping that a shock absorber is to provide, three vehicle performance characteristics are often considered: ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle, as well as the spring constant of the seat, tires, and the damping of the shock absorbers. Vehicle handling is related to the variation in the vehicle's attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces are required to avoid excessively rapid variation in the vehicle's attitude during cornering, acceleration, and deceleration. Road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road holding ability, large damping forces are required when driving on irregular surfaces to prevent loss of contact between the wheels and the ground for an excessive period of time.
Because different driving characteristics depend on the amount of damping forces the shock absorber provides, it is often desirable to have a shock absorber in which the amount of damping forces generated by the shock absorber is adjustable. One method for selectively changing the damping characteristics of a shock absorber is disclosed in European Patent Application Publication No. 0 186 908 A2. In European Patent Application Publication No. 0 186 908 A2, a controller detects the distance between the body of the automobile and the front wheel so as to determine the contour of the surface. A rotary valve in each of the rear shock absorbers is then adjusted so that the rear shock absorbers are able to provide the desired amount of damping forces.
Another method for selectively changing the damping characteristics of a shock absorber is disclosed in PCT International Publication No. WO 88/06983. In PCT International Publication No. WO 88/06983, the shock absorber has a solenoid which controls the flow of damping fluid into pressure chambers which are located adjacent to valve disks which control the damping characteristics of the shock absorber. Upon movement of the plunger of the solenoid, the pressure in these pressure chambers changes so that the damping characteristics of the shock absorber may be varied.
Some of the methods which are used for selectively changing the damping characteristics of a shock absorber require that the displacement of the piston within the pressure cylinder be known. This information is used for determining the velocity and acceleration of the sprung portion of the automobile with respect to the unsprung portion so that the appropriate damping forces may be generated. One method for sensing the position of the piston in the pressure cylinder is disclosed in U. S. Pat. No. 4,788,489. However, this reference requires that both the reserve tube and the piston rod be grounded while the pressure cylinder is electrically insulated from the piston rod and the reserve tube. Furthermore, this reference requires that the piston be made of an electrically insulating material, and that the pressure cylinder be electrically insulated from the reserve tube at a region near the base valve.